1. Field of the Invention
The present invention relates to a force sensor and a motor-driven parking brake apparatus using the force sensor, and is applicable to, for example, a parking brake apparatus for a vehicle.
2. Description of the Related Art
A conventional force sensor includes a first member to which a magnet is fixed; a second member to which an electrical element is fixed, wherein an output voltage of the electrical element varies with relative displacement in relation to the magnet; and a main spring which is interposed between the first member and the second member, and elastically deforms in accordance with force in the reciprocating direction produced between the first member and the second member. International patent application laid-open Nos. 02/20324 (WO 02/20324 A1) and 02/057122 (WO 02/057122 A1) disclose such a force sensor.
Also, these publications disclose a motor-driven parking brake apparatus using the above-described force sensor. The motor-driven parking brake apparatus can be used in a vehicle, and is configured to bring a parking brake into a braking state through rotation of an electric motor in the regular direction, and to bring the parking brake into a released state through rotation of the electric motor in the reverse direction.
In the force sensor described in the publications, along the direction of relative reciprocating movement between the first member and the second member, the first member separably abuts one end portion of the main spring, and the second member separably abuts the other end portion of the main spring. Therefore, even after the amount of elastic deformation of the main spring becomes zero as a result of release of the above-mentioned force, the return movement of the second member with respect to the first member cannot be stopped, and consequently the magnet and the electrical element can still produce a relative displacement therebetween, whereby the output voltage from the electrical element changes continuously. Accordingly, a point in time at which release of the above-mentioned force is completed; i.e., a point in time at which the amount of elastic deformation of the main spring becomes zero, cannot be detected accurately.
In the case where the main spring does not cause permanent set, the point in time at which release of the above-mentioned force is completed can be detected as follows. The force sensor is previously measured for the relation between the amount of elastic deformation of the main spring and the output voltage from the electrical element, and a value of the output voltage from the electrical element at the time the amount of elastic deformation of the main spring becomes zero is previously obtained as a reference value. Through comparison of the actual output voltage from the electrical element with the reference value, the point in time at which release of the above-mentioned force is completed can be detected. However, it is not uncommon for permanent set of the main spring to be generated because of use, and the above-mentioned reference value varies with the permanent set of the main spring. Therefore, although at the beginning of actual use of the sensor the point in time at which release of the above-mentioned force is completed can be detected accurately, in an intermediate or late stage of use, the point in time at which release of the above-mentioned force is completed cannot be detected accurately.
Moreover, in the motor-driven parking brake apparatus using a force sensor disclosed in the above-mentioned publications, even when proper setting is performed at the beginning of use, because of generation of permanent set of the main spring, the sensor becomes unable to accurately detect a time at which the force acting on the parking braking becomes substantially zero, possibly raising various problems in relation to control of operation of the electric motor.